The present invention relates to the removal of liquid hydrocarbons from polyether solvents and especially to the removal of liquid hydrocarbons found in natural gas, synthetic natural gas, synthesis gas and refinery gas which build up in a recirculating polyether solvent when the solvent is used to absorb acid gases from such feedstocks.
A known method of removing acid gases such as hydrogen sulfide, carbon dioxide, COS and methyl mercaptan from the feedstocks is to contact the feed stock at low temperature and high pressure with a polyether solvent such as the dimethyl ether of polyethylene glycol to absorb the acid gases. The liquid is then flashed to remove first absorbed gaseous hydrocarbons and then the bulk of the acid gases. The remaining liquid is called semilean solvent. While some semilean solvent may be recycled to the absorber, some or all of it is generally purified further by stripping with steam or air or other inert gases to remove residual acid gases. In many applications, water present in the semilean solvent can be vaporized by external heating to provide the stripping gas.
Often, however, higher hydrocarbons, and particularly those of 4-8 carbons such as butanes, pentanes, hexanes, heptanes, octanes, benzene, toluene, xylenes and ethylbenzene, are also absorbed in the solvent in increasing amounts in the absorber, even when their concentration in the feedstock gas is relatively low. Because these liquid hydrocarbons are less volatile than methane or other feedstock gases and also less volatile than most acid gases, they will remain predominately in the semilean solvent rather than be flashed off in either step. Lower hydrocarbons such as ethane and propane will also be present to a lesser extent in the semilean solvent. If left in the semilean solvent they can either be stripped in the stripper and/or build up in the solvent returned to the absorber. The former is undesirable if the acid gases are to be used for applications in which hydrocarbon contamination is undesirable, e.g. hydrogen sulfide to be oxidized to sulfur in a Claus plant. The presence of hydrocarbons in hydrogen sulfide fed to a Claus plant causes the resultant sulfur to be brown. Condensing the acid gases to remove the liquid hydrocarbons has the undesirable effect of losing hydrogen sulfide in the condensate, and also requiring energy for both the condensation and subsequent reheating of the acid gases.
Returning the liquid hydrocarbon in the solvent to the absorber has the undesirable effect of increasing the amount of methane, ethane and propane dissolved in the solvent within the absorber and not flashed off. Besides decreasing the fuel value of the purified feed gas, this also results in hydrocarbon contamination of the acid gases recovered in the second and subsequent flashing steps or in stripping steps.
It has been proposed to remove the liquid hydrocarbons especially after cooling, by skimming off the separate layer that forms on the recirculating solvent. Unfortunately, reliance on phase separation alone leaves high levels of liquid hydrocarbons in the solvent. It has been proposed to reduce the residual hydrocarbon content by adding water to the solvent, removing the layer and then removing this water by vaporization. Amounts of water sufficient to appreciably lower hydrocarbon content of the recirculated solvent are so large as to cause an unacceptable amount of energy use for volatilization. Furthermore, unless the water is removed from the solvent, the efficiency of the solvent in absorbing acid gases in the absorber decreases. Thus the need exists for a method of removing liquid hydrocarbons from a polyether solvent without substantially increasing the water content of the solvent.